1. Field of the Invention
This invention relates to ionomeric compositions which have high resilience, and more specifically it relates to such compositions formed from ethylene/methacrylic acid or acrylic acid based copolymers with high levels of acid, neutralized with lithium, zinc and optionally sodium ions. The compositions are particularly useful as golf ball covers.
2. Description of Related Art
Golf ball covers made from ionomers based on ethylene/methacrylic acid or acrylic acid are known in the prior art and have long been used commercially.
Many properties are essential in a cover material. Covers must be adequately tough over a range of temperatures and must be durable. Beyond these essentials however, certain other characteristics of the cover material are desirable. While both the center and windings of a three-piece ball and the core of a two-piece ball are major determinants of the playing characteristics of a golf ball, the cover too plays a significant role. A key property which has long been recognized as a guide to obtaining better cover materials is the resilience of the material. This is often assessed by measurement of the coefficient of restitution (COR).
There is an ever continuing search for materials with the highest possible COR, combined with adequate toughness and durability, for use as covers for golf balls. This search however has revealed divergent, even contradictory trends, or mere broad generalities with regard to which acid, at what level, whether a single metal ionomer or a blend of two three or more metal ionomers should be blended, and how much of each are best, as can be seen from the following references.
U.S. Pat. No. 3,819,768 (Molitor), discloses that when golf balls with the same core are covered with a blend of sodium and zinc ionomer, sodium ionomer alone or zinc ionomer alone, the blend gives a ball with a higher COR than either alone. In other words a synergistic effect is taught when using zinc and sodium together.
Research Disclosure Journal Article #27103, published by Kenneth Mason Publications Ltd., November 1986, and disclosed by E.I. du Pont de Nemours & Company, discloses that lithium ionomers can be used to produce golf ball shells (covers) which can be used to produce golf balls with superior properties, including coefficient of restitution, to shells made of sodium or zinc ionomers. It is also disclosed that lithium ionomers can be used in blends with other ionomers where they can impart better cut resistance to these other materials.
U.S. Pat. No. 5,000,459 (Isaac), in its disclosure interprets the above du Pont disclosure to mean that while lithium ionomers can be used to upgrade the properties of other ionomer resins, the lithium ionomers resins exhibit the best properties if they are used alone. The patent goes on to disclose that blends of lithium and sodium ionomers when used as covers for golf balls produce balls with substantially similar properties to those using lithium ionomer alone. Thus, while no synergism is disclosed on blending, (as is the case with sodium/zinc ionomer blends discussed above), the lithium ionomer can be diluted with sodium ionomer with no deleterious effect. The incentive for doing this is that lithium ionomers are substantially more expensive than sodium ionomers.
Another Research Disclosure Journal Article #29703, published in January 1989, and disclosed by E.I. du Pont de Nemours & Company, discloses that ionomers produced from polymers of ethylene/acrylic or methacrylic acid containing greater that 15 wt. % acid are stiffer and harder than ionomers with less acid. These materials are useful for improving articles where these properties are an advantage, such as golf balls.
Japanese published application No. J63-229077A, published Sep. 22, 1988 discloses blends of ionomers of two different metals or a single ionomer neutralized with two different metal ions. The first metal ion may be an alkali metal ion which is any of lithium, sodium, potassium or cesium and the second metal ion may be zinc or magnesium. In addition, if a single ionomer is used it must be based on ethylene/acrylic acid, (not methacrylic acid) while if a blend of two ionomers is used, the alkali metal ionomer must also be based on an acrylic acid copolymer.
U.S. Pat. No. 4,911,451 (Sullivan et al.), discloses that improved resilience is obtained in a golf ball cover using a blend of a sodium ionomer based on an ethylene copolymer with about 11 wt. % acrylic acid, and a zinc ionomer based on an ethylene copolymer with 11-16 wt. % acrylic acid. In the examples, the average level of acid is well below 16 wt. %. The acid must be acrylic acid, methacrylic acid derived ionomers showing poorer properties. Lithium, magnesium and other metal ions are described as being within the scope of the invention.
Japanese published application No. J61-82768 discloses golf balls with high resilience using a cover material which is a mixture of sodium ionomer or sodium and zinc ionomer with lithium, potassium, cesium or rubidium ionomers.
U.S. Pat. No. 5,068,151 discloses a cover with improved resilience which contains as the major constituent, a lithium ionomer based on an ethylene/alpha,beta-unsaturated carboxylic acid copolymer with 10-20 wt. % acid. Higher than 20 wt. % acid is disclosed as producing a brittle resin. In addition to lithium ionomer, the cover may also contain an ionomer of a bivalent or trivalent metal such as magnesium, calcium, zinc, aluminum, barium and copper, with magnesium and zinc being preferred. Sodium ionomer is not mentioned.
U.S. Pat. No. 4,526,375 (Nakade), discloses that covers of a mixed ionomer of sodium and zinc, sodium and magnesium or preferably sodium, zinc and magnesium have superior low temperature toughness.
Golf balls having a cover which are a blend of lithium and zinc ionomers based on ethylene-acid copolymers containing up to 15% methacrylic acid are also known.
There remains a need for a golf ball cover material having superior resilience, as measured by coefficient of restitution.